One way of making a micro microphone as a microelectomechanical systems (MEMS) device is to produce required vent holes below a metal grid, which is above a resonance cavity. A simple way to create this structure is to etch deep holes through the metal grid, flip the substrate up side down and etch the cavity, (or vice-versa). This uses and Inductively Coupled Plasma (ICP) to etch straight wall holes with Deep Reactive Ion Etching (DRIE) technique. More specifically, one can use the process described in U.S. Pat. No. 5,501,893, or 6,127,273, called Bosch process to produce both processing steps.
However, the main disadvantage with this approach lies in the step where the metal is exposed to the plasma when etching through the metal grid because metal sputtering can impair the properties of the ICP reactor. This causes a long-term drift of the etching characteristics as the metal is sputtered on the chamber walls. The coating of the chamber walls with the sputtered metal causes a parasitic capacitor that changes the impedance of the chamber. This parameter is essential for a good control of the etch profiles.
Other methods were explored as described in the next section; however, they all result in more processing steps and higher costs. Furthermore, standard techniques would require the utilisation of, at least, two masks and two etching steps while the present invention utilizes only one mask to create holes in the bottom of a cavity.
Isotropic and anisotropic plasma etching is widely used to fabricate micro-machined structures and devices. They are generally used separately to remove or define some geometry in a specific layer. The geometries are generally defined with photolithography technique that leaves the image of the photolithographic mask in a photoresist. This pattern is then duplicated by etching one or a few layers under it using different etching techniques.
More specifically, MEMS processes generally use deep reactive ion etching (DRIE) to form structures with high aspect ratios. The geometries are generally critical in such systems, and non-conventional approaches are often utilised to get the desired shape. Different anisotropic techniques were previously invented using different shrewdness to accomplish etch shapes with high aspect ratios. This is either done by carefully adjust etching and passivation species, by switching etching and passivation species alternatively or by using physical bombardment or intrinsic physical properties of the substrate to create a directionality [e.g. TMAH etching].
One object of the present invention is to form an etched pattern in the bottom of a cavity. Various techniques were considered. One such technique is wafer bonding. FIG. 1a to FIG. 1f summarize the process. One would etch a pattern in a substrate (FIG. 1b), etch a cavity in a second substrate (FIG. 1d) and then, create a permanent contact bonding between the two substrates with a prior alignment to fix the cavity at the desire position relative to the pattern. This solution is expensive since two substrates are needed and the cavity substrate often needs to be back grinded to get the desired thickness or to open the cavity. The later leads into a depth of the cavity difficult to control and leads to large variation across the wafer.
A second technique uses the spray coating technique. It starts first with an anisotropic or isotropic etch to create the desire cavity (FIG. 2a to 2b), then is followed by the removal of the etching mask (FIG. 2c). Then, a spray coater is used to coat the walls and the bottom of the cavity with photoresist (FIG. 2d). A photolithography mask aligner able to project the photolithography mask image in the bottom of the cavity without distortion (with a large depth of field) is then used to create the pattern in the bottom of the cavity (FIG. 2e). Finally, DRIE (Deep Reactive Ion Etch) is used to imprint the pattern in the bottom of the cavity (FIG. 2f). This technique is greatly limited in the depth of the cavity. Greater the depth of the cavity worse is the resolution of the pattern in the bottom of the cavity.
A third technique is possible when the pattern in the bottom of the cavity is extended throughout the whole substrate. This can be accomplished by creating the cavity on one side of the substrate with the desired depth (FIG. 2d), and then the substrate is flipped over and a second mask is patterned on the other side (aligned with the first one: FIG. 3a). Finally, the DRIE is used to etch the pattern and reach the cavity (FIG. 3b). Either the cavity or the pattern can be done first. This solution implies handling of the two side of the wafer and the thickness of the substrate need to be very uniform. Furthermore, when etching the second side (either the cavity or the pattern), at the point where the cavity depth reach the “bottom” of the etched pattern or vice versa, lost of dimensions is generally seen. This can be due to lost of the backside cooling if using DRIE. It leads to erosion of the pattern. When etching the cavity in second, sidewall passivation of the pattern (left over by DRIE or protective layer to preserve the dimensions) can create micro-masking leading to unwanted re-entrant contour walls in the cavity. This is explained in U.S. Pat. Nos. 6,500,348 and 6,685,844 where they propose solution to minimize this effect.
If the patterned holes do not need to breakthrough the wafer, a second wafer substrate is needed, with optional back grinding if a specific thickness is needed.
All above techniques need many processing steps to create etched patterns in the bottom of a cavity. This leads to high costs and pour throughput for the overall process.